Conductivity, temperature, and depth measurements were carried out in an isolated transverse lead in static, shorefast ice in Mould Bay, Prince Patrick Island, Northwest Territories, during a 3-week period at the height of the melt season. Currents beneath the ice appeared to be weak and largely tidal in nature. Initially, the water was vertically uniform and at the salinity-determined freezing point down to a depth of at least 20 m. By the end of the experiment the water column consisted of three distinct layers: a well-mixed, nearly fresh surface meltwater layer; a very stable half-meter-thick halocline centered somewhat below the button of the ice; and a thermally stratified layer of constant salinity extending down to at least 25m. The halocline was characterized by a temperature maximum that was about 2¿C warmer than the surrounding water. This temperature maximum in the pycnocline effectively trapped shortwave energy absorbed in the lower layer and prevented if from melting the overlying ice. Theoretical calculations demonstrate that the thermal structure observed beneath the pycnocline was controlled by the input of shortwave radiation and that the thermal structure observed beneath the pycnocline was controlled by the input of shortwave radiation and that vertical heat transport was largely the result of diffusive processes. The presence of leads drastically increases the amount of energy stored in the water. In regions where leads are common, it is likely that this energy will significantly accelerate the decay and removal of the ice once it becomes mobile and once the pycnocline is erased. ¿American Geophysical Union 1990